1. Field of the Invention
This invention relates to a synchronizing arrangement for a picture display device, comprising a synchronizing signal separating stage in which horizontal synchronizing pulses are recovered from a television signal which contains a horizontal synchronizing composite signal. The horizontal synchronizing pulses are applied to the synchronizing arrangement and a horizontal synchronizing circuit receives the horizontal synchronizing pulses for synchronizing purposes.
In synchronizing arrangements of this type, which as a rule serve for the purpose of synchronizing the deflection unit of a picture display device with the received television signal, the problem occurs that when there is a lot of noise in the television signal, or when the television signal fails to arrive, the horizontal synchronizing circuit, more specifically a phase-locked loop which is usually present, wanders within its pull-in range, i.e. it does not produce a horizontal synchronizing signal of a constant frequency. As a result, stable picture gating operations are then not possible or the television picture, on which noise is superposed, is additionally disturbed by a faulty synchronization.
2. Description of Related Art
European Patent Specification 0135236 which corresponds to U.S. Pat. No. 4,636,860 (Jan. 13, 1987) discloses a picture display arrangement having a vertical synchronizing circuit, which is provided with a counter arrangement which, in a predetermined time interval, counts the number of times in which the amplified signal with the superimposed noise exceeds a predetermined value. If this happens too often, the mode of operation of certain stages of the picture display device are changed-over. More particularly, the sensitivity of a PLL, which produces the vertical synchronizing signals for a deflection unit is changed. In this arrangement, however, it is a condition that in spite of the prevailing noise, it must still be possible to unambiguously identify the synchronizing pulses. If this is not the case anymore, then it is also impossible for the phase-locked loop to operate correctly and the above-described problems occur.